The velocity of propagation of an envelope produced when an electromagnetic wave is modulated by, or mixed with, other waves of different frequencies. Note: The group velocity is the velocity of information propagation and, loosely, of energy propagation. 2. In optical fiber transmission, for a particular mode, the reciprocal of the rate of change of the phase constant with respect to angular frequency. Note: The group velocity equals the phase velocity if the phase constant is a linear function of the angular frequency, = 2f, where f is the frequency. 3. In optical-fiber transmission, the velocity of the modulated optical power.